Active matrix liquid crystal displays (AMLCD) have eliminated many problems associated with passive displays. For example, the fabrication of active matrix liquid crystal displays have enabled display screens to achieve greater brightness, enhanced readability, a greater variety of color shades, and broader viewing angles compared to displays that employ other technologies. Active matrix liquid crystal displays generally comprise an array of picture elements called pixels. An electronic switch is associated with each pixel in the display to control the operation thereof. Various electronic switches such as, for example, thin film transistors and organic light emitting diodes (OLED), among others have been investigated to control pixel operation. Thin film transistors, in particular, offer a high degree of design flexibility and device performance.
Thin film transistors generally are formed on large area substrates having a high degree of optical transparency such as, for example, glass substrates. FIG. 1 depicts a cross-sectional schematic view of a thin film transistor (TFT) 122 being a type that has a bottom gate structure. The thin film transistor 122 includes a glass substrate 101 having an underlayer 102 formed on the surface thereof. A gate is formed on the underlayer 102. The gate comprises a gate metal layer 104 and a gate dielectric 108. The gate controls the movement of charge carriers in the transistor. The gate dielectric 108 formed over the gate metal layer 104 electrically isolates the gate metal layer 104 from semiconductor layers 110, 114a, 114b, formed thereon, each of which may function to provide charge carriers to the transistor. A source region 118a of the transistor is formed on semiconductor layer 114a and a drain region 118b of the transistor is formed on semiconductor layer 114b. Finally, a passivation layer 120 encapsulates the thin film transistor 122 to protect it from environmental hazards such as moisture and oxygen.
Each layer is critical with respect to the electrical performance of the thin film transistor (TFT). In particular, the gate dielectric layer needs to have certain qualities (e.g., low flatband voltage (Vfb)) in order for the transistor to have overall desirable electrical parameters, such as, for example, a high breakdown voltage (VB).
Many film layers can be deposited using conventional techniques, such as, for example, plasma assisted chemical vapor deposition (PECVD). Unfortunately, high temperatures are required to deposit film layers using PECVD techniques and high deposition temperatures may not be compatible with some substrates, such as glass substrates, as the glass may soften and become dimensionally unstable.
Therefore, a need exists to develop a method of forming high-quality film layers on temperature-sensitive substrates.
A method of film deposition is described herein. The film is deposited using a cyclical deposition process. The cyclical deposition process consists essentially of a substantially continuous flow of one or more process gases modulated by alternating periods of pulsing and non-pulsing where the periods of pulsing alternate between pulsing a precursor into the process environment, and pulsing energy into the process environment to generate a plasma. Thus, the methods consist essentially of placing a substrate in a process chamber; exposing the substrate to a substantially continuous flow of a process gas composition under process conditions, and providing a period of non-pulsing. Next, a pulse of a precursor is provided to the process environment. Under the process conditions, the precursor does not react with the process gas composition. Once the precursor has been provided to the process environment, a second period of non-pulsing is provided. Next, a high frequency power is provided to the process environment to produce a plasma. Under the plasma conditions, the process gas composition does react with the precursor. The reaction produces a film layer. The steps of pulsing and non-pulsing are repeated until a desired thickness of the film layer has been formed.
The methods of the present invention eliminate the need to provide a carrier gas and/or a purge gas in addition to a reactant gas unlike prior art methods. Instead, a xe2x80x9cprocess gasxe2x80x9d or xe2x80x9cprocess gas compositionxe2x80x9d is provided to a chamber in a continuous or substantially continuous manner throughout the deposition of the desired film layer. Essentially any film layer can be deposited in this manner using any precursor and process gas combination, as long as the precursor and the process gas composition do not react with each other (or react minimally) under process conditions, but do react with each other when the process environment is supplied with enough energy to produce a plasma.